We studied the effect of selective inhibition of the neural isoform of nitric oxide synthase in the rat renal medulla in conscious Sprague-Dawley rats. Continuous renal medullar interstitial infusion of an antisense oligonucleotide complementary to the initiation region of the mRNA for neural nitric oxide synthase increased blood pressure 14 +/- 1 mm Hg in rats maintained on a high sodium intake. Medullary interstitial infusion of saline vehicle or a scrambled oligonucleotide probe failed to alter blood pressure in separate groups of high salt control rats. Renal medullary interstitial infusion of the antisense oligonucleotide significantly decreased the level of neural nitric oxide synthase in the renal medulla by 53 +/- 8% and decreased total renal medullary nitric oxide synthase activity by 28 +/- 8%. No alterations were detected in the levels of inducible nitric oxide synthase or beta-actin in the antisense oligonucleotide-infused rats. To confirm the antisense oligonucleotide data, we administered a mechanistically different inhibitor of neural nitric oxide synthase, 7-nitroindazole, to an additional group of rats maintained on a high salt diet. Direct renal medullary interstitial infusion of this selective enzyme inhibitor significantly increased mean arterial pressure (15 +/- 6 mm Hg) and decreased total renal medullary nitric oxide synthase activity by 37 +/- 12% in rats on a high sodium diet. The present experiments demonstrate a role for the neural isoform of nitric oxide synthase in the long-term control of blood pressure in the presence of a high salt diet.